The present invention relates to a hydraulic anti-skid brake system of a motor vehicle, and more particularly to a system for controlling the power train of the vehicle, when the anti-skid brake system (ABS) is in operation.
The anti-skid brake system is installed on a brake system for wheels of the vehicle. The anti-skid brake system is provided for preventing the wheels from completely locking, and hence from skidding during rapid braking or by braking on slippery surfaces such as snowy roads, thereby ensuring directional stability and steering control of the vehicle during the braking operation and improving safety of the vehicle. For example, Japanese Patent Laid Open 60-61354 discloses an anti-skid brake system where an anti-skid control unit is operated to stop a supply of brake fluid to cylinders of the wheels so as to reduce the brake fluid pressure in dependency on speed reduction rate of the wheels and on relative reduction rate of the wheels to that of the vehicle speed. When the wheel speed is increased by the reaction from the road surface to approach the vehicle speed, the brake fluid is then again supplied to the cylinders to increase the pressure. The operation is repeated so as to effectively brake the vehicle without locking the wheels.
However, on a road having little reaction or a small friction coefficient, the wheels are easily locked with a slight braking pressure. Although the anti-skid brake system is operated to reduce the fluid pressure in the cylinders, since the reaction of the road surface is small, it requires a long time for the wheels to recover their speed. Moreover, although a minimum pressure is applied after the wheel speed is recovered, braking effect is too large for the road surface to cause an excessive slipping rate which results in unstable steering of the vehicle. Such a phenomenon is aggravated when engine braking is effected on the vehicle body.
If the engine braking is effected in the anti-skid operation, the engine braking force is exerted on the wheels to lock them in spite of the anti-skid operation. Since the anti-skid brake system operates to reduce the fluid pressure in the system in accordance with the deceleration of the wheel speed, if the wheel speed is disturbed by the engine braking, the efficiency of the anti-skid brake system is reduced. Thus, it is desired to eliminate the disturbance due to the operation of the engine braking, for providing a proper anti-skid brake system control.
In order to eliminate the disturbances, there has been proposed a system where a transmission control unit is provided in the anti-skid brake system to disconnect a clutch provided in the power train between a transmission of the vehicle and the wheels. Namely the power transmitting system from the engine to the wheels is disconnected when the anti-skid brake system is operated. The disconnection of the transmitting system, where the engine braking does not act, continues as long as the anti-skid brake system operates. However, exclusive terminals for operating the transmission control unit must be provided on the anti-skid control unit. Accordingly, a conventional anti-skid brake system can not be used.